Click here to purchase the entire book in PDF format. Acoustic Impedance CHANGE ALL THIS? Before we look at how rooms behave when you make noise in them, we have to begin by looking at the concept of acoustic impedance. Earlier, we saw how sound is transmitted through air by moving molecules bumping up against each other. One air molecule moves and therefore moves the air molecules sitting next to it. In other words, we're talking about energy being transferred from one molecule to another. The ease with which this energy is transferred between molecules is measured by the difference in the acoustic impedances of the two molecules. I'll explain. We've already seen that sound is essentially a change in pressure over time. If we have a static barometric pressure and then we apply a new pressure to the air molecules, then we change their displacement (we move them) and create a molecular velocity. So far, we've looked at the relationship between the displacement and the velocity of the air molecules, but we haven't looked at how both of these relate to the pressure applied to get the whole thing moving. In the case of a pendulum (a weight hanging on the end of a stick that's free to swing back and forth), the greater the force applied to it, the more it moves and the faster it will go - the higher the pressure, the greater the displacement and the higher the velocity. The same is true of the air molecules - the higher the pressure, the greater the displacement and the higher the velocity. However, the one thing we're ignoring is how hard it is to get the pendulum (or the molecules) moving. If we apply the same force to two different pendulums, one light one and one heavy one, then we'll get two different maximum displacements and velocities as a result. Essentially, the heavier pendulum is harder to move, so we don't move it as far. The issue that we're now discussing is how much the pendulum impedes your attempts to move it. The same is true of molecules moved by a sound wave. Air molecules are like a light pendulum - they're relatively easy to move. On the other hand, if we were to put a loudspeaker in poured concrete and play a tune, it would be much harder for the speaker to move the concrete molecules - therefore they wouldn't move as far with the same pressure applied by the loudspeaker. There would still be a sound wave going through the concrete (just as the heavy pendulum would move - just not very much) but it wouldn't be very loud. The measurement of how much velocity results from a given amount of pressure is an indication of how hard it is it move the molecules - in other words, how much the molecules impede the transfer of energy. The higher the impedance, the lower the velocity for a given amount of pressure. This can be seen in Equation 3.12 which is true only for the free field situation. $$z = \frac{p}{u}$$ (4.12) where z is the acoustic impedance in acoustic ohms (abbreviated $\Omega$). As you can see in this equation, $z$ is proportional to $p$ and inversely proportional to $u$. This means that if the impedance goes up and the pressure stays the same, then the velocity will go down. In the specific case of unbounded plane waves (waves with a flat wavefront - not curved like the ones we're been discussing so far), this ratio is also equal to the product of the volume density of the medium, $\rho_o$ and the speed of wave propogation $c$ as is shown in Equation 3.13 [Olson, 1957]. This value $z_{o}$ is known as the specific acoustic impedance or characteristic impedance of the medium. $$z_{o} = \rho_{o} c$$ (4.13) MORE???